AC Auxillary Fan
 

My 85 300D AC works great most of the time. Blows cold, but when I idle, particularly in hot weather, it stops blowing cold. Am copncerned about aux fan. Yesterday it was blowing warm and the aux fan was not on. What makes it come on and when does this happen? I beleive there is a switch/relay at the front right of engine area. Could thois be bad? Any suggestions on how to diagnose the problem.

thanks:)

Bob:

Two things to check -- there is a pressure switch on the receiver/dryer, I think, that switches the fan on when the head pressure comes up -- should cycle the fan on low speed. Low speed is obtained by using the resistor (rectangular ceramic resistor somewhere near the receiver/dryer, I think). If the switch or resistor is bad, no fan.

The other is fan operation -- pull the wire on the switch on the thermostat housing and ground it with the ignition on -- fan must run at high speed. If not, check relay and fuse. If these seem OK, connect fan directly to a battery at the connection up front with jumper wires (have a care, the wires can get HOT) -- fan must run.

MB aux fans rarely go bad -- most likely you have a bad resistor, bad switch or relay, or corroded fuse.

Peter

You're barking up the wrong tree. The problem you describe is typically fixed by replacing the fan clutch on the engine driven fan. To test it, turn off the engine and give the fan a good, swift twirl. If it turns more than 1/2 turn, you need to replace the fan clutch.

Good luck,

Aux fan helps you as a supplemental cooling mechanism. Even if it does not work the AC should work but the engine will run hotter on temp scale. Like LB said make sure the main fan clutch is fine. You will find tons of topics on that.

If your AC is cooling the hoses in the engine compartment but still blows hot air in cabin, its probably one of the five switch over valves that you have behind the ACC control panel. Look for my earlier threads if its indeed the case.

I agree with LB that you are trouble shooting in wrong direction.


MVK

hmmmmm.... and I thought that the aux fan helps air flow through the evaporator, and when it does not work the refrigerant temp goes up along with the pressure and that shuts off the ac compressor to avoid a blow up... go figure...

On my car Aux fan comes on only when engine gets too hot. not as soon as you press AC on. But I still get the cooling without any blowing up of AC. But feel free to correct me If i am wrong. I and not and expert AC guy.

Thanks

MVK

Auxiliary fan not working
 

I had a problem with the auxiliary fan also. After thoroughly checking everything I determined the following:

1) Fan works if 12V is applied direct from the battery.

So, the problem must be the relay. Get new relay and install it. Jump the switch and guess what: Still no fan.

Take the relay out and apply 12V to the terminal that goes to the fan. Fan works.

So, now what?

Check for voltage at the relay at the terminal supply contact and find that it is reading 6V!:confused:

How is this possible?

With no other ideas, what the hell, check the fuse.

I find the fuse corroded to the terminal ends. Remove fuse, replace with new fuse

Problem solved, fan works.:D

How to replace the auxiliary fans on ML320 (2000)
 

Hi All,

New here. The auxiliar fans on my ML320 stopped working and the engine runs hot. The mechanic wants to replace it for $625 (parts and labor). I found the parts for $154 on the web. My question is, how difficult it is to replace the fans by myself? Is there a manual somewhere? I would love to do this myself and save few hundreds.:smhair:

Thanks.

Intfin

Old thread, revived for a gas ML?

psfred was spot-on.

Anyway, on the 201 and 124, the aux fan is turned on by:

High engine coolant temperature (105C?), ... high fan speed.

Refrigerant (A/C) pressure (?? bar) during normal operation, ... low fan speed through resistor.

If it doesn't run during A/C operation, you might have low refrigerant pressure. Apply 12v directly to one end of the rectangular ceramic (speed) resistor: If it runs high speed with 12v applied to one end, but doesn't run with 12v on the other end, the resistor is bad. Common. If it runs high speed with 12v on one end, and low speed with 12v on the other end, the resistor is fine.

With the aux fan(s) not running, your A/C will not cool well when driving slow or idling, will cool down at highway speeds normally.

I've had this symptom in one 124 and one 201, in both cases it was a bad speed resistor and was cured by replacing the resistor. The engine fan is for cooling the engine, not the A/C condensor, the electric fans are for cooling the condensor, and for cooling the engine at low speeds.

There are two pressure switches on the A/C: one is the low-pressure cutout that cuts power to the compressor if there is iinadequate refrigerant pressure as tf007 mentioned, the other is the high-pressure switch that turns the fan on low speed.

And yes, the temperature switch on the engine runs the fan directly.

i installed a small relay so the fan runs when the ac compresser is on.

Every car i have had operates this way.

This seems to work and is an easy fix to the fan no work problem.

Steve

A 1985 300D is neither a W124 nor a W201. Nor does it operate the same way.

What is the opposite of "spot-on?" "Spot-off?"

an old thread yes but what type relay?
 

So did you run a line with a fuse in it directly from the compressor 12 volt wire and your sharing the 12v supply from the compressor to the fans?
Have you had any problems with that? like blowing fuses or the compressor 12v line getting too hot?


I'm considering doing what you did due to my compressor wont get cold until my fans come on but they only come on at idle when I drive the fans go off the compressor still runs but air is not much cold. At idle with fans blowing the digital thermometer is reading 12Faren. at the interrior vents.

In case fireman is not still here to tell me then maybe this other symptom will solicit some help..see the new to me ml air was not working when I got it. I replaced rec/dryer and TXV and I evac the system of 134a then vacuumed then refilled. The compressor was coming on but not getting cooled air in cabin. So checking the reading I felt the pressure maybe a little too high so I let out a little Freon and fans switched on and started cooling then higher rpms cuts the fan off. From reading posts about the high and low switches it just does not sound like replacing either would help, so I don't know which way to go. The thought does prod me that maybe let out a little more Freon but I hate to get too low and risk starting all over again.
Firemans workaround sounds favorable though if I'm reading him correctly.

Not sure how fireman did it, but I produce a relay that does exactly that. However, I am working on a dual relay setup that uses two separate fuses/relays that turn on both the compressor clutch and the cooling fan whenever the CCU calls for A/C. This way, both the clutch and the fan have their own full 12v and fuse......Rich

My original search was for a ml320.
But seeing now this is a diesel discussion.
My fault,guess I need to find the correct category and post my problem.

:o

When can we hope to get our hands on your new relay and wiring configuration kit?